Magnetic disk unit having anti-impact arrangement

ABSTRACT

A magnetic disk unit is provided which can protect internal mechanisms from external force. A cover 11 covers the outer periphery of a base 12 which supports a spindle mechanism 2 and a head positioning mechanism 5. The cover 11 is connected to the base 12 elastically to reduce the possibility that the external force directly acts on the base and to assure that the external force acting on the cover is transmitted to the base via a cushioning material, whereby the spindle mechanism and the head positioning mechanism are protected from the external force.

TECHNICAL FIELD

The present invention relates to a magnetic disk unit and, inparticular, to a portable and compact magnetic disk unit that can bedetached from an information processing apparatus and carried.

BACKGROUND ART

To make a magnetic disk unit compact and thinned so that it may be usedas a portable and exchangeable storage medium, an internal spindlemechanism, a magnetic head positioning mechanism and so on must beprotected from impacts that result from falls or the like of themagnetic disk unit.

For example, Japanese Unexamination Patent Publication No. 5-258545discloses a magnetic disk unit in which a viscous member is sandwichedbetween a base and a top plate to absorb vibrations of its cover.However, the disclosure fails to propose any solution to avoid anadverse effect caused to a spindle motor and a head positioner byimpacts exerted to the base plate.

Further, Japanese Unexamination Patent Publication No. 4-368690discloses a magnetic disk unit in which the four external corners of thecase of the unit are provided with elastic members so as to weakenimpacts caused when the unit falls and hits against a floor. However,the disclosure has proposed no solution to cope with impacts exerted tothe unit at its portions other than the four corners.

DISCLOSURE OF THE INVENTION

As described above, the prior art magnetic disk unit lacks solutions toprotect the spindle motor, the head positioner and so on from impactswhich are frequently exerted in horizontal directions to the portableand compact magnetic disk unit when it is handled during carrying orfrom impacts exerted to the magnetic disk unit when it is placed on adesk top.

Accordingly, it is an object of the present invention to provide amagnetic disk unit that is capable of protecting internal mechanismsfrom impacts exerted in a wider range of directions.

It is another object of the present invention to provide a magnetic diskunit that is capable of weakening the impact that acts on the base tothereby protect internal mechanisms such as a spindle mechanism and ahead positioning mechanism mounted on a base.

It is a further object of the present invention to provide a thin typemagnetic disk unit that is capable of protecting its internal mechanismsagainst fall and collision.

According to the first aspect of the invention, there is provided amagnetic disk unit comprising a spindle mechanism for rotating amagnetic disk, a head positioning mechanism for positioning a magnetichead by shifting said magnetic head along a recording surface of saidmagnetic disk, a base supporting said spindle mechanism and said headpositioning mechanism, and a cover that covers in a non-contactingrelationship said spindle mechanism and said head positioning mechanismsupported on said base, wherein the joint portion of the cover and thebase is structured such that the lower edge portion of said coverextends downward beyond the level of the underside of said base to coverthe outer peripheral edge of said base in a non-contacting relationshipthereto and said cover and said base are connected together in the innerportion of said cover with a cushioning material interposedtherebetween.

According to the second aspect of the invention, there is provided amagnetic disk unit comprising a spindle mechanism for rotating amagnetic disk, a head positioning mechanism for positioning a magnetichead by shifting said magnetic head along a recording surface of saidmagnetic disk, a base supporting said spindle mechanism and said headpositioning mechanism, and a cover that covers in a non-contactingrelationship said spindle mechanism and said head positioning mechanismsupported on said base, wherein said head positioning mechanism supportssaid magnetic head such that said magnetic head is movable in a spacebetween said magnetic disk and said base, the joint portion of the coverand the base is structured such that the lower edge portion of saidcover extends downward beyond the level of the underside of said base tocover the outer peripheral edge of said base in a non-contactingrelationship thereto and said cover and said base are connected togetherin the inner portion of said cover with a cushioning material interposedtherebetween.

According to the third aspect of the invention, there is provided amagnetic disk unit comprising a spindle mechanism for rotating amagnetic disk, a head positioning mechanism for positioning a magnetichead by shifting said magnetic head along a recording surface of saidmagnetic disk, a base supporting said spindle mechanism and said headpositioning mechanism, and a cover that covers in a non-contactingrelationship said spindle mechanism and said head positioning mechanismsupported on said base, wherein the joint portion of the cover and thebase is structured such that the lower edge portion of said coverextends downward beyond the level of the underside of said base to coverthe outer peripheral edge of said base in a non-contacting relationshipthereto and said cover and said base are connected together in the innerportion of said cover with a cushioning material interposedtherebetween, and said base is provided with openings in its innercorners along the outlines thereof.

Since the cover extends to cover the outer peripheral edge of the base,the possibility that an external force is directly applied to the baseis reduced. Since the external force that acts on the cover istransmitted to the base via a cushioning material (cushioningstructure), undue external force is prevented from acting on the spindlemechanism and the head positioning mechanism, whereby dislocation isavoided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional side view of the magnetic disk unitaccording to a first embodiment of the present invention.

FIG. 2 is an enlarged view of a part of FIG. 1.

FIG. 3 is an enlarged view showing dimensional relation of the magneticdisk unit shown in FIG. 1.

FIG. 4 is a longitudinal sectional side view of the magnetic disk unitaccording to a second embodiment of the present invention.

FIG. 5 is a perspective view of the outline of the magnetic disk unitshown in FIG. 4.

FIG. 6 is a longitudinal sectional side view of the magnetic disk unitaccording to a third embodiment of the present invention.

FIG. 7 is an enlarged view of a part of FIG. 6.

FIG. 8 is a perspective view of the outline of the magnetic disk unitaccording to a fourth embodiment of the present invention.

FIG. 9 is a perspective view of the outline of the magnetic disk unitaccording to a fifth embodiment of the present invention.

FIG. 10 is a longitudinal sectional side view of the magnetic disk unitaccording to a sixth embodiment of the present invention.

FIG. 11 is an enlarged view of a part of FIG. 10.

FIG. 12 is an exploded perspective view of the magnetic disk unitaccording to a seventh embodiment of the present invention.

FIG. 13 is an enlarged view showing an example of the joint portion ofthe embodiment shown in FIG. 12.

FIG. 14 is an enlarged view showing another example of the joint portionof the embodiment shown in FIG. 12.

FIG. 15 is a perspective view of the outline of the magnetic disk unitaccording to an eighth embodiment of the present invention.

FIG. 16 is a sectional view along line A--A in FIG. 15.

FIG. 17 is a longitudinal sectional side view showing the assemblyprocess of the magnetic disk unit according to a ninth embodiment of thepresent invention.

FIG. 18 is a longitudinal sectional side view showing the assemblyprocess of the magnetic disk unit according to a tenth embodiment of thepresent invention.

FIG. 19 is an exploded perspective view of the magnetic disk unitaccording to an eleventh embodiment of the present invention while FIG.19a is a cross-sectional view of a portion of such embodiment.

FIG. 20 is a perspective view of the outline of the magnetic disk unitaccording to a twelfth embodiment of the present invention.

FIG. 21 is a longitudinal sectional side view of the magnetic disk unitshown in FIG. 20.

BEST MODES FOR CARRYING OUT THE INVENTION

FIGS. 1 through 3 show the first embodiment of the present invention,wherein FIG. 1 is a longitudinal sectional side view of the magneticdisk unit, FIG. 2 is an enlarged view of a part thereof, and FIG. 3 isan enlarged view showing dimensional relationship thereof.

A spindle mechanism 2 with a magnetic disk 1 mounted thereon is fixed toan upper surface of a base 12 made of a stainless steel sheet. Thespindle mechanism 2 rotates the magnetic disk 1 about a spindle centeraxis 2a.

A head positioning mechanism 5 has, on the end of its suspension arm 5a,a magnetic head 3 which reads information from or writes information onthe recording surface of the magnetic disk 1 and is mounted on the uppersurface of the base 12. Furthermore, the head positioning mechanism 5 ispivoted about its axis of rotation 5b so that the magnetic head 3 movesin the space formed between the magnetic disk 1 and the base 12 andalong the recording surface formed on the underside of the magnetic disk1.

A cover 11 made of aluminum having a lower edge portion 11a which isslightly larger than the peripheral edge of the base 12 is provided withan upwardly offset stepped surface 11b inward of the lower edge portionof the cover 11. The base 12 has its outer peripheral edge fixed to thestepped surface 11b. A space is provided between the outer peripheraledge of the base 12 and the inner peripheral face of the lower edgeportion 11a of the cover 11 to accommodate a cushioning material 8a madeof rubber, and a bolt-connection to the stepped surface 11b is madethrough rubber cushioning materials 8b, 8c interposed therebetween. Thediameters of bolt holes 12a formed in the base 12 are larger than theouter diameters of the bolts 10 which extend therethrough and arescrewed into the stepped surface 11b, so that the bolts are innon-contacting relation to the base. The cushioning material 8b is sizedto leave a small clearance between the stepped surface 11b of the cover11 and the base 12 when the bolts 10 are tightened at a predeterminedforce to secure the base 12 to a part of the cover 11 inward of thelower edge portion 11a of the cover 11, so that any impact acting on thecover 11 is prevented from being directly transmitted to the base 12.With the stepped surface 11b of the cover 11 being taken as a referencelevel, the cover 11 is internally dimensioned such that a cover portionadjacent the spindle mechanism 2 mounted on the base 12 is of a level11d that is higher than the height 2b of the spindle mechanism 2,another cover portion adjacent the head positioning mechanism 5 mountedon the base 12 is of a level 11e that is higher than the height 5c ofthe head positioning mechanism 5, whereby the spindle mechanism 2including the magnetic disk 1 and the head positioning mechanism 5 donot contact the cover 11.

Since, according to the magnetic disk unit thus constructed, the cover11 covers the spindle mechanism 2 and head positioning mechanism 5mounted on the base 12 and the outer peripheral edge of the base 12 andfurther extends downward beyond the base 12, collision of the spindlemechanism 2 or the head positioning mechanism 5 when the unit falls oris transported is prevented, and the chance of collision of the base 12is decreased. Since spaces or cushioning materials 8a through 8c areinterposed, respectively, between the spindle mechanism 2 and the headpositioning mechanism 5, and between the base 12 that supports both thespindle mechanism 2 and head positioning mechanism 5 and the cover 11,any impact exerted to the cover 11 will not be directly transmitted tothe spindle mechanism 2 and head positioning mechanism 5, so that theyare protected against such impact.

FIGS. 4 and 5 show the second embodiment of the present invention. FIG.4 is a longitudinal sectional side view of the magnetic disk unit, andFIG. 5 is a perspective view of the outline thereof. In comparison withthe above-described magnetic disk unit of the first embodiment, thefeature of this embodiment lies in the dimension of the overallthickness of the unit being 10.5 mm (or less) to meet the type IIIstandard of IC memory card. The rest of the construction remainsunchanged from the first embodiment, so that any further description isomitted.

The magnetic disk unit of such dimensions allows the prior artinformation processing apparatus using an IC memory card as an externalmemory device to be easily modified to be for use with the magnetic diskunit, so that an information processing apparatus may also be soconstructed as to be compatible with both the IC memory card and themagnetic disk unit.

The above dimensions may be implemented into the preceding embodimentand third to eleventh embodiments to be described later, as well.

FIGS. 6 and 7 show the third embodiment of the invention. FIG. 6 is alongitudinal sectional side view of the magnetic disk unit and FIG. 7 isan enlarged view of a part thereof. Those components of the embodimentidentical or equivalent to those described with reference to the firstembodiment are designated by the same reference numerals so as to savedetailed explanation thereof.

In comparison with the first embodiment already described, the featureof this embodiment is that the joint between the cover 11 and the base12 is achieved by adhesives 17a, 17b on both sides of the cushioningmaterial 8d disposed between the cover 11 and the base 12.

This embodiment is characterized in that the cushioning and joiningbetween the cover 11 and the base can be realized by a small number ofcomponent parts and by a simple joining operation. Such a jointstructure using the adhesive may be applied to any of the first andsecond embodiments, as well as to any of the fourth to twelfthembodiments to be described later.

FIG. 8 is a perspective view of the magnetic disk unit according to thefourth embodiment of the present invention. The feature of thisembodiment is that the cover 11 is made of a plastic material with shockabsorbers 9a through 9d bonded to the corners of the cover 11. Thisembodiment may be applied to any of the first to third embodiments andany of the fifth to twelfth embodiments by modification.

According to this embodiment, the cover 11 can be manufactured byplastic molding at low costs to achieve reduction in manufacturing costand in weight. Furthermore, since the corners of the unit that aresubject to impacts when the unit is carried or falls are protected bythe shock absorbers 9a through 9d, the weakening of impacts andprevention of damage are achieved to increase the reliability of theunit.

FIG. 9 is the perspective view of the magnetic disk unit according tothe fifth embodiment of the present invention. In comparison with eachof the preceding embodiments, the feature of this embodiment is that thecover 11 of a molded plastic has a viewing window 11f in an area opposedto the range of pivotal motion of the suspension arm 5a and the magnetichead 3 so that the conditions of the magnetic disk 1, magnetic head 3and suspension arm 5a can be viewed from outside. The viewing window 11fis fan-shaped and formed of a transparent material. This embodiment isapplicable to any of the already described embodiments by modification.The modification must include a structure that the magnetic head 3 andthe suspension arm 5a are disposed in the space between the magneticdisk 1 and the cover 11 to allow the magnetic head 3 and the suspensionarm 5a to be viewed from outside the cover 11.

In order that the magnetic head 3 and the suspension arm 5a disposed inthe space between the magnetic disk 1 and the base may be viewed, asimilar viewing window may be formed in the base 12.

This embodiment may be applied to any of the sixth to twelfthembodiments to be described later.

FIGS. 10 and 11 show the seventh embodiment of the present invention.FIG. 10 is a longitudinal sectional side view of the magnetic disk unit,and FIG. 11 is a enlarged view thereof. In this embodiment, on its lowerend, the lower edge portion 11a described with reference to the thirdembodiment is provided with a seal application area 11g which isrecessed to the level flush with the underside 12b of the base 12 sothat the clearance between the cover 11 and the base 12 can be sealinglyclosed by a sealing tape 13 adhesively applied to the seal applicationarea 11g and the underside 12b of the outer peripheral portion of thebase 12.

The joining and sealing between the cover 11 and the base 12 achieved byadhesive advantageously reduce the number of the component parts and themanufacturing steps for joining operation.

The sealed structure using the sealing tape 13 may be applied to any ofthe first, second, fourth and fifth embodiments already described and toany of the seventh to twelfth embodiments to be described later.

FIGS. 12 through 14 show the seventh embodiment of the presentinvention. FIG. 12 is an exploded perspective view of the magnetic diskunit and FIGS. 13 and 14 are enlarged cross-sectional views.

The feature of this embodiment is that the base 12 and the cover 11 arejoined by elastically deforming the corners of the base 12 intoengagement with the inside of the lower edge portion 11a of the cover11. The corners of the base 12 have openings 12c through 12f formedalong the corner outlines, so that the peripheries of the corners thatdefine the openings 12c through 12f can be elastically deformed intoengagement with the inside of the lower edge portion 11a of the cover11.

In the joint structure shown in FIG. 13, elastic deformation is made ofthe corners of the base 12 so that a recess 11h formed in the inner wallthat extends downward from the stepped surface 11b of the lower edgeportion 11a of the cover 11 receives the thus deformed corners of thebase 12. Opening 12c for imparting elasticity to a corner of the base 12is, in its engaged state, sealed by a cushioning material 8e interposedbetween the stepped surface 11b of the cover 11 and the base 12.

In the joint structure shown in FIG. 14, the under surface of the loweredge portion 11a is provided with a seal application area 11g that isrecessed to the level flush with the underside 12b of the base 12 sothat a sealing tape 13 can be applied to the seal application area 11gand the underside 12b of the outer periphery of the base 12 to close theopening 12c to assure that the cover 11 and the base 12 are preventedfrom being dislodged and sealing is achieved.

The joining by the engagement that is achieved by the elasticdeformation of the corners of the base 12 where the openings 12c through12f are formed is effective to reduce the transmission of an impactacting on the cover 11 to the base 12 as well as to decrease the numberof the component parts and the manufacturing steps for the joiningoperation.

FIGS. 15 and 16 show the eighth embodiment of the present invention.FIG. 15 is a perspective view, and FIG. 16 is a longitudinal sectionalview taken along line A--A in FIG. 15. The feature of this embodiment isthat the cover 11 of the already described first embodiment is modifiedso that posts 11j, 11k extend therefrom toward the base 12 with elasticmembers 8f, 8g applied to the ends of the posts and ribs 11m, 11n areformed to extend radially from the positions of the posts 11j, 11k todecrease the deflection of the cover 11.

The cover 11 in this embodiment can be formed by plastic molding. Theelastic members 8f, 8g are bonded to the ends of the posts 11j, 11k.When the cover 11 and the base 12 are in their assembled state, theelastic members 8f, 8g contact the base 12 to form elastic spacermembers. As for the joint structure, any structure described withreference to each of the preceding embodiments and embodiments to bedescribed later may be employed.

In the magnetic disk unit thus constructed, external force applied tothe cover 11 when the unit is carried is absorbed by means of therigidity of the ribs 11m and 11n and deformation of the elastic members8f, 8g caused by the posts 11j, 11k, so that the amount of deflection ofthe cover 11 can be minimized. Even when a relatively large externalforce is exerted to the cover 11, therefore, the cover 11 can beprevented from being deflected into interfering engagement with thespindle mechanism 2 and with the head positioning mechanism 5.

The posts 11j, 11k, the spacers formed by the elastic members 8f, 8g andthe rigidity reinforcements by the ribs 11m, 11n employed in thisembodiment may be applied to any of the other embodiments.

FIG. 17 is a longitudinal sectional side view showing the assemblyprocess of the magnetic disk unit according to the ninth embodiment ofthe present invention. In this embodiment, the magnetic disk unit has amagnetic disk 1 whose top surface is formed as a recording surface. Aspindle mechanism 2 which rotates a magnetic disk 1 is mounted on a base12 first, and then a head positioning mechanism 5 is assembled. A cover11 is then mounted on the base 12 to cover the spindle mechanism 2 andthe head positioning mechanism 5 which have already been attached to thebase. Then, bolts 10 are tightened to secure the members together withthe cushioning materials 8b interposed between the stepped surface 11bof the lower edge portion 11a and the base 12.

FIG. 18 is a longitudinal sectional side view showing the assemblyprocess of the magnetic disk unit according to the tenth embodiment ofthe present invention. In this embodiment, the magnetic disk unit has amagnetic disk 1 whose bottom surface is formed as a recording surface. Ahead positioning mechanism 5 is first mounted on a base 12, and then aspindle mechanism 2 which rotates the magnetic disk 1 is mounted on thebase 12. A cover 11 is then mounted on the base 12 to cover the spindlemechanism 2 and the head positioning mechanism 5 which have already beenattached to the base. Then, bolts 10 are tightened to secure the memberstogether with a cushioning material 8b interposed between the steppedsurface 11b of the lower edge portion 11a and the base 12.

FIG. 19 is an exploded perspective view of the magnetic disk unitaccording to the eleventh embodiment of the present invention, whileFIG. 19a is a cross-sectional view of a portion of such embodiment. Inthis embodiment, a base 12 is in the form of an oblong plate having aspindle mechanism seating section 12g on one end and a head positioningmechanism seating section 12h on the other end with a line extendingbetween both seating sections included within the base 12. A spindlemechanism 2 and a head positioning mechanism 4 are mounted on thespindle mechanism seating section 12g and the head positioning mechanismseating section 12h, respectively. The base 12 is secured to a plasticbase retaining plate 14 via a cushioning material 8h. The base 12 isthen connected to the inside of a lower edge portion 11a of a plasticcover 11. The base retaining plate 14 is of a rigidity-reinforcedstructure having embossed portions 14a and openings 14d definingplastically deformable base portions which are plastically deformed soas to engage portions of the impact cover to lock them together.

In the magnetic disk unit thus constructed, any impact exerted to thecover 11 is absorbed by deflections of the cover 11 and the baseretaining plate 14 and the cushioning material 8h, so that the base 12can be protected against deformation. The cover 11 has a downwardextending portion 11g (FIG. 19a) extending downward below a level of anunderside of the base retaining plate 14. A seal 13 is also provided.Relative displacement between the spindle mechanism 2 and the headpositioning mechanism 5 can therefore be prevented to improve thereliability.

FIGS. 20 and 21 show the twelfth embodiment of the present invention.FIG. 20 is a perspective view of the outline of the magnetic disk andFIG. 21 is a longitudinal sectional side view thereof. The feature ofthis embodiment is that the overall dimension of the unit in thedirection of its thickness is 3.3 mm or less to meet the type I standardof IC memory card. The rest of the structure remains unchanged from thatof the first embodiment, so that any further description is omitted.

A magnetic disk unit of such dimension allows the prior art informationprocessing apparatus using an IC memory card as an external memorydevice to be easily modified to be for use with the magnetic disk unit,whereby an information processing apparatus may be constructed so as tobe compatible with both the IC memory card and the magnetic disk unit.

The above dimension may be applied to any of the first and third toeleventh embodiments already described.

In the present invention, the cover extends over the outer peripheraledge of the base that supports the spindle mechanism and the headpositioning mechanism mounted thereon and the cover is connected to thebase elastically to lessen the chance that an external force directlyacts on the base and to cause the external force to be transmitted tothe base via cushioning means, whereby the invention provides a smallsized magnetic disk unit which is capable of protecting its internalmechanisms mounted on the base.

We claim:
 1. A magnetic disk unit comprising a spindle mechanism forrotating a magnetic disk, a head positioning mechanism for positioning amagnetic head by shifting said magnetic head along a recording surfaceof said magnetic disk, a base supporting said spindle mechanism and saidhead positioning mechanism, and an impact cover that covers in anon-contacting relationship said spindle mechanism and said headpositioning mechanism supported on said base,wherein said base includesan oblong base plate and a base retaining plate for restraining saidbase plate, a joint portion of said base plate and said base retainingplate contains a cushioning material therein wherein said base retainingplate extends to a lower edge portion of said impact cover and is fittedinto and connected to said impact cover in an inner portion of saidimpact cover, and said base retaining plate is provided with openingsdefining plastically deformable base portions which are plasticallydeformed so as to engage portions of said impact cover to lock said baseretaining plate and said impact cover together.
 2. The magnetic diskunit according to claim 1, wherein an overall thickness of the unit isnot greater than 10.5 mm.
 3. The magnetic disk unit according to claim1, wherein an overall thickness of the unit is not greater than 3.3 mm.4. The magnetic disk unit according to claim 1, wherein at least one ofsaid impact cover and said base retaining plate is provided with aviewing window to observe said magnetic disk and said head positioningmechanism from outside.
 5. The magnetic disk unit according to claim 1,further comprising a seal connected to both said impact cover and saidbase to seal the joint portion therebetween.
 6. A magnetic disk unitcomprising a spindle mechanism for rotating a magnetic disk, a headpositioning mechanism for positioning a magnetic head by shifting saidmagnetic head along a recording surface of said magnetic disk, a basesupporting said spindle mechanism and said head positioning mechanism,and an impact cover that covers in a non-contacting relationship saidspindle mechanism and said head positioning mechanism supported on saidbase,wherein said base includes an oblong base plate and a baseretaining plate for restraining said base plate, a joint portion of saidbase plate and said base retaining plate contains a cushioning materialtherein, wherein said base retaining plate extends to a lower edgeportion of said impact cover, said impact cover extends downward to havea lip substantially flush with a level of an underside of said basecover to cover an outer peripheral edge of said base cover in order toprovide impact protection thereto, said base retaining plate is fittedinto and connected to said impact cover in an inner portion of saidimpact cover, and said base retaining plate is provided with openingsdefining plastically deformable base portions which are plasticallydeformed so as to engage portions of said impact cover to lock said baseretaining plate and said impact cover together.
 7. The magnetic diskunit according to claim 6, wherein an overall thickness of the unit isnot greater than 10.5 mm.
 8. The magnetic disk unit according to claim6, wherein an overall thickness of the unit is not greater than 3.3 mm.9. The magnetic disk unit according to claim 6, wherein at least one ofsaid impact cover and said base retaining plate is provided with aviewing window to observe said magnetic disk and said head positioningmechanism from outside.
 10. The magnetic disk unit according to claim 6,further comprising a seal connected to both said impact cover and saidbase to seal the joint portion therebetween.
 11. A magnetic disk unitcomprising a spindle mechanism for rotating a magnetic disk, a headpositioning mechanism for positioning a magnetic head by shifting saidmagnetic head along a recording surface of said magnetic disk, a basesupporting said spindle mechanism and said head positioning mechanism,and an impact cover that covers in a non-contacting relationship saidspindle mechanism and said head positioning mechanism supported on saidbase,wherein said base includes an oblong base plate and a baseretaining plate for restraining said base plate, a joint portion of saidbase plate and said base retaining plate contains a cushioning materialtherein, wherein said base retaining plate extends to a lower edgeportion of said impact cover, said impact cover extends downward beyonda level of an underside of said base cover to cover an outer peripheraledge of said base cover in order to provide impact protection thereto,said base retaining plate is fitted into and connected to said impactcover in an inner portion of said impact cover, and said base retainingplate is provided with openings defining plastically deformable baseportions which are plastically deformed so as to engage portions of saidimpact cover to lock said base retaining plate and said impact covertogether.
 12. The magnetic disk unit according to claim 11, wherein anoverall thickness of the unit is not greater than 10.5 mm.
 13. Themagnetic disk unit according to claim 11, wherein an overall thicknessof the unit is not greater than 3.3 mm.
 14. The magnetic disk unitaccording to claim 11, wherein at least one of said impact cover andsaid base retaining plate is provided with a viewing window to observesaid magnetic disk and said head positioning mechanism from outside. 15.The magnetic disk unit according to claim 11, further comprising a sealconnected to both said impact cover and said base to seal the jointportion therebetween.